Field of the Invention
The present invention generally relates to the field of power cables for electric power transmission and distribution, in particular to cables for underground and submarine use.
Description of the related art
Power cables for power transmission in the low voltage (up to 1 kV) and particularly medium voltage (MV) and high voltage (HV) ranges (from 1 kV to 35 kV for MV and higher than 35 kV for HV, respectively) typically comprise one or more insulated conductors (one insulated conductor for single-phase power transmission, three insulate conductors for 3-phase power transmission; cables with a single insulated conductor are also referred to as “single-core” cables, while cables with more than one insulated conductor are also referred to as “multi-core” cables). With “insulated conductor” it is meant an electrical conductor surrounded at least by a polymeric insulating layer. An inner and/or an outer semiconducting layer can be disposed around the conductor and the insulating layer, respectively. An armour, for example of metal wires or metal tapes, can be provided to surround each core for providing resistance to tensile stress. A polymeric sheath is typically provided as outermost layer.
When a power cable has to be installed in a wet or potentially wet environment, such as underground or underwater, its insulated conductor/s should be protected from humidity penetration that may lead to electrical breakdown. For this purpose, the cable is equipped with a water barrier, which can be provided around each insulated conductor and/or around the bundle of insulated conductors for preventing or at least reducing the occurrences of water trees possibly producing electrical breakdown.
Different types of water barrier are known. For example, T. Worzyk, Submarine Power Cables, Power Systems, Springer-Verlag Berlin Heidelberg 2009, pages 30-33 discloses that aluminium, lead, copper, and other metals can be used as water-blocking sheath in a variety of shapes. Laminated aluminium sheaths consisting of a thin aluminium foil pre-laminated with a layer of PE-copolymers can be formed around the cable core with the polymeric layer outside. The edges of the laminate strip are glued together with overlap. Tiny amounts of humidity might diffuse into the cable through the glued seam.
U.S. Pat. No. 4,221,926 discloses waterproof cables of the type having a metallic shield formed with a longitudinal seam wrapped about a cable core. In particular, the cable is formed with an outer shield composed of a strong metal to provide the cable with structural strength. The outer shield is wrapped about the core in a generally cylindrical configuration with one longitudinal edge overlaying another longitudinal edge thereby forming a shield seam. The shield seam is overlayed with a strip of tape, typically a laminated structure composed of aluminium foil, Kraft paper, and Mylar film, or, alternatively, of polyolefin, such as a spunbonded, high density polyethylene fibrous material or a biaxially oriented polypropylene film.
Simple, cheap and versatile water barrier implementations particularly suited to underground cables (both of the “buried” and of the “in vault” types) make use of metallic foils or polylaminate foils. A polylaminate foil is a multi-layer foil comprising at least a first layer of metal material, such as aluminium or copper, and a second layer of polymer material, such as polyethylene. For example, U.S. Pat. No. 3,575,748 discloses a cable jacket for electrical cables formed by a tape folded longitudinally around the core of the cable. In the tape there is a metal foil laminate with plastic coating on both its upper and lower surface.
According to known solutions, metal and polylaminate water barriers are manufactured by wrapping a metal or polylaminate foil around the core/s of the cable up to overlap opposite edges thereof one to each other. The wrapped foil is fastened around the core/s by providing a bonding agent, e.g. comprising polymeric adhesive resin, between the overlapping edges thereof.
US 2007/0194085 discloses a low temperature joining method that is compatible with multiple materials and results in a fusion between joined structures without reducing the mechanical properties of the joined structure's base materials. The method includes the steps of contacting a first structure to a second structure; and directing particles of a metallic bonding material towards an interface between the first structure and second structure at a velocity to cause the particles of the metallic bonding material form a molecular fusion between the first structure and second structure.